As a conventional type of a rotational speed control system for an electric motor which is referred to as a motor hereinafter, a time sharing servo system has been proposed and is well known. The time sharing servo system employs a switching regulator in which a counter electromotive force is detected for producing a rotational speed control signal while electric power is not supplied to a motor. The above-mentioned method of rotational speed control is proposed by a U.S. Pat. No. 3,624,474.
According to the above-mentioned method, a train of rectangle pulse signals is produced by an astable multivibrator where the signal train has a given repetative frequency and a duty-cycle. The motor of the system is fed with an electric power while the output of the astable multivibrator is high (H) or low (L). The counter electromotive force produced by the motor is detected during one of above-mentioned periods for which the electric power is not supplied and the rate of supplying the electric power to the motor is regulated in accordance with the magnitude of the detected counter electromotive force.
This means that the motor is supplied with the electric power fully in the feeding period when the rotational speed of the motor is below a predetermined rotational speed while the motor is supplied with the electric power with a lower supplying rate than before when the rotational speed of same is over the predetermined rotational speed.
The above-mentioned method is called a time sharing servo method which utilizes a PWM (pulse with modulation) signal since electric power is fed to the motor upon presence of a pulse where the pulse width varies in accordance with the variation of the load of the motor. The periods of feed and non-feed of the electric power are almost equal to each other. The time sharing servo method utilizing a PWM signal described hereinabove, however, includes some disadvantages which will be described hereinbelow.
The ratio of a period for which the electric power may be fed to the motor to the other period for which the electric power is not fed to the same is determined by the duty cycle of the train of pulses produced by the astable multivibrator. Therefore the ratio of same is constant. Assuming the ratio is 3, that is the period for which the electric power may be fed to the motor is 3 and the period for which the electric power is not fed to same is 1, the maximum feeding ratio can not be arranged over 3 to 1 even though the rotational speed of the rotor of the motor is below a predetermined value. Therefore the electric power fed from the power supply is utilized by the motor to an extent of three fourths maximum causing a fact that one fourth of same is not utilized at all.
In the time sharing servo method the non-feed period is utilized for detecting the counter electromotive force and this period of time for detecting the counter electromotive force can not be made so short that the counter electromotive force may be influenced by spike pulses produced by the motor immediately after the electric power is cut after each feed period. The peak value of the above-mentioned spike pulse depends on a differentiation of the current through the armature of the motor in time as well as the inductance of the armature. Therefore, the counter electromotive force can not be detected during a period of time for which the spike pulse maintains.
As an example, the inductance of small DC motor utilized for a cassette tape recorder is approximately 10 to 20 mH and a spike pulse produced by same lasts approximately 0.5 to 1.0 msec. If the period of time for which the spike pulse lasts is over 1 msec, the counter electromotive force can not be detected when the period of time for detecting same is arranged less than 1 msec. Therefore the detecting period should be approximately 10 msec in order to provide an accurate control.
Turning back to the before-mentioned ratio of the feed and non-feed of the electric power, where the ratio is 3 to 1, a period of time of one cycle of feed and non-feed is 40 msec and thus the motor is regulated by a PWM switching signal of 25 Hz. When it is aimed to raise the efficiency of the feed of the electric power, the above-mentioned ratio may be arranged as 9 to 1 so that 90% of the electric power is utilized by the motor. With this arrangement, however, the switching frequency becomes 10 Hz with which the electric power is fed to the motor only once in three rotations of the rotor of the motor when the rotor is arranged to rotate at 1,800 rpm. In this manner of feed of electric power, vibration of the motor is apt to increase and further the accuracy of the control is also apt to deteriorated.
As described above according to the conventional time sharing servo method, it is required to lengthen the feed period in order to increase the efficiency of the feed of the electric power where the long period of feed compared to that of non-feed causes a decrease of the switching frequency since the minimum detecting time for detecting the counter electromotive force is limited depending of the characteristic of the motor.